Optical tomographic imaging apparatus and method for controlling the same

ABSTRACT

An optical tomographic imaging apparatus includes a control unit configured to, in a case where an instruction to narrow an imaging range of a tomographic image is issued, control a measurement light optical path length changing unit to reduce an optical path length of measurement light and make a changing speed of the optical path length of the measurement light lower than in a case where an instruction to widen the imaging range is issued.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical tomographic imagingapparatus and a method for controlling the same. For example, thepresent invention relates to an optical tomographic imaging apparatusfor use in ophthalmic practice and a method for controlling the same.

2. Description of the Related Art

Optical image measurement techniques for forming an image of the surfaceand/or inside of an object to be measured by using light are attractingattention in recent years. Unlike conventional X-ray computed tomography(CT), the optical image measurement techniques are not invasive to thehuman body. Applications of the optical image measurement techniques areexpected to be developed especially in the medical field. Significantprogress has been made in the ophthalmological field in particular.

Among typical techniques for optical image measurement is a methodcalled optical coherence tomography (OCT). This method uses aninterferometer, which enables high-resolution high-sensitivitymeasurement. Using wideband weak light as illumination light provides anadvantage of high safety to a subject.

An optical tomographic imaging apparatus based on OCT (hereinafter,referred to as an OCT apparatus) using optical interference can obtain atomographic image of a sample with high resolution. In particular, theOCT apparatus relates to an anterior eye optical tomographic imagingapparatus for forming an image of an anterior eye of a subject's eye.

The OCT apparatus can irradiate a sample with low coherent light servingas measurement light, and measure backscattered light from the samplewith high sensitivity by using an interference system or an interferenceoptical system. The OCT apparatus can scan the sample with themeasurement light to obtain a high-resolution tomographic image. Atomographic image of a cornea region of the anterior eye of a subject'seye can thus be obtained and used for ophthalmic diagnosis.

Japanese Patent Application Laid-Open No. 2011-147612 discusses anoptical tomographic imaging apparatus that can capture both atomographic image of an anterior eye and a tomographic image of afundus. According to whether an imaging mode is an anterior eye imagingmode or a fundus imaging mode, the optical tomographic imaging apparatusmoves a reference mirror included in its interference optical system toa position corresponding to the imaging mode.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an optical tomographicimaging apparatus configured to obtain a tomographic image of an objectbased on light into which return light from the object irradiated withmeasurement light and reference light corresponding to the measurementlight are combined includes a measurement light optical path lengthchanging unit configured to change an optical path length of themeasurement light, an instruction unit configured to issue aninstruction on a size of an imaging range of the tomographic image, anda control unit configured to, in a case where an instruction to narrowthe imaging range is issued, control the measurement light optical pathlength changing unit to reduce the optical path length of themeasurement light and make a changing speed of the optical path lengthof the measurement light lower than in a case where an instruction towiden the imaging range is issued.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an entire optical tomographic imagingapparatus according to a first exemplary embodiment.

FIG. 2 is a diagram illustrating a configuration of a measurementoptical system according to the first exemplary embodiment.

FIG. 3 is an explanatory diagram illustrating a state where an anterioreye of a subject's eye is scanned in an x direction according to thefirst exemplary embodiment.

FIGS. 4A, 4B, and 4C are explanatory diagrams illustrating scan rangesin an imaging position of an anterior eye according to the firstexemplary embodiment and images obtained according to the scan ranges.

FIG. 5 is a diagram illustrating an example of a measurement operationscreen according to the first exemplary embodiment.

FIG. 6 is a diagram illustrating another example of the measurementoperation screen according to the first exemplary embodiment.

FIG. 7 is a flowchart illustrating a measurement flow according to thefirst exemplary embodiment.

FIGS. 8A, 8B, 8C, 8D, 8E, and 8F are diagrams illustrating displayexamples of tomographic images of an anterior eye according to the firstexemplary embodiment and display examples of the images corrected.

FIG. 9 is a diagram illustrating an example of a measurement operationscreen according to second to fourth exemplary embodiments.

DESCRIPTION OF THE EMBODIMENTS

Take the case of changing the size of the imaging range of a tomographicimage of an object such as a subject's eye. A possible method mayinclude changing the optical path length of measurement light by movingthe apparatus main body with respect to the object in an optical axisdirection. With such a method, an operator cannot readily know how muchthe optical path length of the measurement light is to be changed toobtain a tomographic image of the size intended by the operator.

An aspect of an exemplary embodiment is directed to easily obtaining atomographic image of the intended size by the operator specifying thesize of the imaging range of the tomographic image of the object.Another aspect of the exemplary embodiment is directed to improvingsafety by facilitating the operator determining whether the apparatusmain body possibly comes into contact with a subject's eye.

An optical tomographic imaging apparatus according to the presentexemplary embodiment can, if an instruction on a size of an imagingrange of a tomographic image is issued, change an optical path length ofmeasurement light according to the instruction. As a result, theoperator can easily obtain a tomographic image of the intended size byissuing an instruction on the size of the imaging range of thetomographic image of an object. If an instruction to narrow the imagingrange of the tomographic image is issued, the optical tomographicimaging apparatus according to the present exemplary embodiment cancontrol a measurement light optical path length changing unit to reducethe optical path length of the measurement light (for example, bring anoptical unit including an optical path of the measurement light closerto the object) and make a changing speed of the optical path length ofthe measurement light lower than if an instruction to widen the imagingrange is issued. As a result, the operator can easily determine whetherthe apparatus main body possibly comes into contact with a subject'seye. This can improve safety.

The imaging range can be narrowed to obtain a detailed enlarged image.The imaging range can be widened to obtain a reduced image over a widerange. The optical path length of the measurement light can be reduced,for example, by moving an optical head closer to the subject's eye alongits optical axis. This reduces the distance between the optical head andthe subject's eye.

An optical tomographic imaging apparatus (OCT apparatus) according to afirst exemplary embodiment will be described below with reference to thedrawings.

[General Configuration of Apparatus]

A general configuration of the optical tomographic imaging apparatusaccording to the present exemplary embodiment will be described withreference to FIG. 1. FIG. 1 is a side view of the optical tomographicimaging apparatus. The optical tomographic imaging apparatus 200includes an optical head 900 which includes a measurement optical systemfor capturing a two-dimensional image and a tomographic image of ananterior eye. A stage unit 950 is a moving unit that can move theoptical head 900 in x, y, and z directions in the diagram by usingnot-illustrated motors. A base unit 951 includes a spectroscope to bedescribed below. The optical head 900, an example of an optical unitincluding an optical path of measurement light, is a housing of themeasurement optical system. The stage unit 950 is an example of anoptical unit moving mechanism that moves with respect to an object.

A personal computer 925 constructs a tomographic image. The personalcomputer 925 also serves as a control unit of the stage unit 950 andcontrols the stage unit 950. A hard disk 926 stores a program fortomographic imaging. The hard disk 926 also serves as a subjectinformation storage unit. A monitor 928 serves as a display unit. Aninput unit 929 is used to issue instructions to the personal computer925. Specifically, the input unit 929 includes a keyboard and a mouse. Achin rest 323 fixes the chin and forehead of a subject to prompt thesubject to fix the eyes (subject's eyes). An external fixation lamp 324is used to fix the subject's eyes. The external fixation lamp 324 and aninternal fixation lamp to be described below can be switched and used.

[Configuration of Measurement Optical System and Spectroscope]

A configuration of the measurement optical system and the spectrometeraccording to the present exemplary embodiment will be described withreference to FIG. 2.

The interior of the optical head 900 will be described. Objective lenses101-1 and 101-2 are located opposite a subject's eye 100. A reflectingmirror 102 and a dichroic mirror 103 are arranged on an optical axis ofthe objective lenses 101-1 and 101-2. By the reflecting mirror 102 andthe dichroic mirror 103, light from the object lenses 101-1 and 101-2 isbranched into optical paths L1 and L2 of respective different wavelengthbands. The optical path L1 is an optical path of an OCT optical system.The optical path L2 is intended for anterior eye observation and for aninternal fixation lamp.

The optical path L2 is further branched by a third dichroic mirror 104into optical paths to a charge-coupled device (CCD) 105 for anterior eyeobservation and an internal fixation lamp 106 according to respectivewavelength bands like described above. Lenses 101-3, 107, and 108 arearranged on the optical path L2. A not-illustrated motor drives the lens107 for the purpose of a focusing adjustment intended for the internalfixation lamp 106 and anterior eye observation. The CCD 105 hassensitivity to wavelengths of not-illustrated anterior eye observationillumination light. Specifically, the CCD 105 has sensitivity towavelengths around 780 nm. The internal fixation lamp 106 generatesvisible light and prompts eye fixation of the subject.

The optical path L1 constitutes the OCT optical system as describedabove. The optical path L1 is intended to capture a tomographic image ofan anterior eye 100-1 of the subject's eye 100. More specifically, theoptical path L1 is intended to obtain an interference signal for forminga tomographic image. A lens 101-4, a mirror 113, an X scanner 114-1, a Yscanner 114-2, and lenses 115 (OCT focus lens 115) and 116 are arrangedon the optical path L1. The X scanner 114-1 and the Y scanner 114-2 areintended to scan the anterior eye 100-1 of the subject's eye 100 withlight. Light from a light source 118 is emitted from a fiber 117-2connected to a photocoupler 117. A not-illustrated motor drives the lens115 to focus and adjust the light emitted from the fiber 117-2 on theanterior eye 100-1. By such a focusing adjustment, light from theanterior eye 100-1 is also incident on and forms a spot-like image onthe end of the fiber 117-2. The lens 115, also referred to as an OCTfocus lens, is an example of a focusing lens.

A configuration of optical paths from the light source 118, a referenceoptical system, and the spectroscope will be described.

The light source 118, a reference mirror 119, dispersion compensationglass 120, the photocoupler 117 described above, single mode opticalfibers 117-1 to 117-4 integrally connected with the photocoupler 117, anlens 121, and the spectroscope 180 constitute a Michelsoninterferometer.

The light emitted from the light source 118 passes through the opticalfiber 117-1 and is split into measurement light on the side of theoptical fiber 117-2 and reference light on the side of the optical fiber117-3 through the photocoupler 117. The measurement light passes throughthe optical path of the OCT optical system described above. The fundusof the subject's eye 100 to be observed is irradiated with themeasurement light. The measurement light is reflected and scattered bythe retina, and passes through the same optical path to reach thephotocoupler 117.

The reference light passes through the optical fiber 117-3, the lens121, and the dispersion compensation glass 120 to reach the referencemirror 119. The dispersion compensation glass 120 is inserted to adjustdispersion of the reference light to that of the measurement light. Thereference light is reflected by the reference mirror 119 and returnsthrough the same optical path to reach the photocoupler 117. Thephotocoupler 117 combines the measurement light and the reference lightinto interference light. Interference occurs when an optical path lengthof the measurement light and that of the reference light satisfy apredetermined condition. The reference mirror 119 is supported to beadjustable in the optical axis direction by a not-illustrated motor anda not-illustrated drive mechanism. The optical path length of themeasurement light varies depending on the anterior eye 100-1. Thereference mirror 119 can adjust the optical path length of the referencelight to that of the measurement light. The interference light is guidedthrough the optical fiber 117-4 to the spectroscope 180.

The spectroscope 180 includes lenses 181 and 183, a diffraction grating182, and a line sensor 184. The interference light emitted from theoptical fiber 117-4 is converted into generally parallel light throughthe lens 181. The generally parallel light is spectrally dispersed bythe diffraction grating 182, and focused on the line sensor 184 by thelens 183. The line sensor 184 is an example of a light receiving elementthat receives the interference light and generates and outputs an outputsignal according to the interference light in the present exemplaryembodiment.

Next, the light source 118 will be described. The light source 118 is asuper luminescent diode (SLD), which is a typical low coherent lightsource. The light source 118 has a center wavelength of 855 nm and awavelength bandwidth of approximately 100 nm. The bandwidth is animportant parameter since the bandwidth influences resolution of theresulting tomographic image in the optical axis direction. While an SLDis selected as the light source 118, any type of light source that canemit low coherent light may be used. Examples include an amplifiedspontaneous emission (ASE) device. In view of eye measurement, nearinfrared light has a suitable center wavelength. Since the centerwavelength influences the resolution of the resulting tomographic imagein a lateral direction, the center wavelength can be as short aspossible. From both reasons, the center wavelength of 855 nm isemployed.

In the present exemplary embodiment, a Michelson interferometer is usedas the interferometer. A Mach-Zehnder interferometer may be usedinstead. Which interferometer to use may be determined according to adifference in light intensity between the measurement light and thereference light. If the difference in light intensity is large, aMach-Zehnder interferometer can be used. If the difference in lightintensity is relatively small, a Michelson interferometer can be used.

[Method for Obtaining Tomographic Image]

A method for obtaining a tomographic image by using the opticaltomographic imaging apparatus will be described. The optical tomographicimaging apparatus can obtain a tomographic image of a desired region ofthe anterior eye 100-1 of the subject's eye 100 by controlling the Xscanner 114-1 and the Y scanner 114-2.

FIG. 3 illustrates a state where the subject's eye 100 is irradiatedwith measurement light 201 and the anterior eye 100-1 is scanned in thex direction. The line sensor 184 captures information about apredetermined number of images from an imaging range of the anterior eye100-1 in the x direction. A fast Fourier transform (FFT) is performed ona luminance distribution on the line sensor 184 obtained in a positionin the x direction. A linear luminance distribution obtained by the FFTis converted into density or color information for monitor display. Suchdensity or color information will be referred to as an A scan image.According to the output signal obtained from the interference lightreceived by the line sensor 184 serving as the light receiving element,the optical tomographic imaging apparatus obtains A scan images. Theplurality of A scan images is arranged into a two-dimensional image,which will be referred to as a B scan image. After the plurality of Ascan images for constructing a B scan image is obtained, the opticaltomographic imaging apparatus moves the scan position in the y directionand performs a scan in the x direction again. In such a manner, theoptical tomographic imaging apparatus obtains a plurality of B scanimages. The plurality of B scan images or a three-dimensionaltomographic image constructed from the plurality of B scan images isdisplayed on the monitor 928. The operator can use the displayedimage(s) to diagnose the subject's eye 100.

The angle of view or the imaging range for obtaining a tomographic imageof the anterior eye 100-1 is usually determined according to a scanrange R0 in the x direction illustrated in FIG. 4A to be describedbelow. The scan range R0 is determined by a scan angle θ of the Xscanner 114-1 and an imaging distance P0 from the objective lens 101-1to the anterior eye 100-1 of the subject's eye 100. In other words, tochange the size of the imaging range, the scan angle θ or the imagingdistance P0 can be changed. The imaging distance P0 can be easilychanged by changing the optical path length of the measurement light,such as by moving the optical head 900 in the z-axis direction. In thepresent exemplary embodiment, the imaging distance P0 is changed bychanging the optical path length of the measurement light of the opticalhead 900. Such a configuration will be defined as a measurement lightoptical path length changing unit. There are other configurations forchanging the optical path length of the measurement light than that ofthe present exemplary embodiment. The measurement light optical pathlength changing unit according to the present exemplary embodiment isdefined as a concept covering such configurations.

To obtain a desired interference by combining the measurement light andthe reference light, the optical path length of the measurement lightand the optical path length of the reference light need to beinterlocked to satisfy a predetermined condition as described above.According to the optical path length of the measurement light in theposition of the anterior eye 100-1 where the imaging distance is P0, thereference mirror 119 is thus moved to change the optical path length ofthe reference light.

The reference mirror 119 and a configuration for moving the referencemirror 119 are an example of a reference light optical path lengthchanging unit for changing the optical path length of the referencelight according to the present exemplary embodiment. As described above,to obtain interference by the combined light, the optical path length ofthe reference light needs to be changed according to the optical pathlength of the measurement light. For example, in the present exemplaryembodiment, the personal computer 925 includes a module area thatfunctions as a control unit (also referred to as an “optical path lengthinterlocking unit”). The control unit causes the reference light opticalpath length changing unit to change the optical path length of thereference light in an interlocking manner with the change of the opticalpath length of the measurement light by the measurement light opticalpath length changing unit.

FIGS. 4A, 4B, and 4C illustrate diagrams illustrating the scan ranges inthe position of the anterior eye 100-1 when the imaging distance P0 ischanged, and corresponding tomographic images displayed in therespective angles of view. By changing the imaging distance P0 andmoving the reference mirror 119 according to the change, the opticaltomographic imaging apparatus can change the size of the imaging rangeof the anterior eye 100-1 without changing the scan angle θ. FIG. 4Billustrates a case where the imaging distance P0 is changed to Pmax toincrease the distance between the subject's eye 100 and the opticaltomographic imaging apparatus, and the reference mirror 119 is moved toa position equivalent to the imaging distance Pmax. In such a case, theanterior eye 100-1 can be imaged with a wide scan range (angle of view)Rmax. FIG. 4C illustrates a case where the imaging distance P0 ischanged to Pmin to reduce the distance between the subject's eye 100 andthe optical tomographic imaging apparatus, and the reference mirror 119is moved to a position equivalent to the imaging distance Pmin. In sucha case, the anterior eye 100-1 can be imaged with a magnifying scanrange Rmin.

[Measurement Operation Screen]

A measurement operation screen according to the present exemplaryembodiment will be described with reference to FIGS. 5 and 6. FIG. 5 isa diagram illustrating an example of a measurement operation screen 1000according to the present exemplary embodiment. FIG. 6 is a diagramillustrating another example of the measurement operation screen 1000according to the present exemplary embodiment.

An anterior eye observation screen 1101 displays an anterior eye image1102 obtained by the CCD 105 for anterior eye observation. A tomographicimage display screen 1301 is intended to check a tomographic imageobtained. L and R buttons 1001 are intended to switch between subject'sleft and right eyes. The L and R buttons 1001 are pressed to move theoptical head 900 to initial positions for the left and right eyes,respectively. When the operator operates the mouse included in the inputunit 929, a position of a mouse cursor 1002 moves. This opticaltomographic imaging apparatus is configured so that a mouse cursorposition detection unit can change an alignment unit according to theposition of the mouse cursor 1002. The mouse cursor position detectionunit calculates the position of the mouse cursor 1002 from a pixelposition of the mouse cursor 1002 on-screen. Ranges are set on themeasurement operation screen 1000, and correspondence between the setranges and alignment drives is set in advance. If the mouse cursor 1002falls within the pixels of a set range, alignment defined for the setrange can be performed. Alignment operations by the mouse are performedby rotating a wheel of the mouse.

Sliders 1103 and 1203 arranged near the respective images are intendedfor adjustment. The slider 1103 is intended to specify the imagingdistance P0 to the subject's eye 100. When the slider 1103 is moved, acharacter 1003 in the anterior eye observation screen 1101 changes insize in an interlocking manner. The size of the character 1003 is alsointerlocked with a change in the size of the imaging range (angle ofview) of the anterior eye 100-1, whereby the lens 107 for anterior eyeobservation is moved to a predetermined position. The lens 107 is anexample of an anterior eye observation unit including a focus lens thatperforms focusing on the anterior eye 100-1 according to the presentexemplary embodiment. An upper limit of the slider 1103 corresponds tothe imaging range Rmax of the anterior eye 100-1 described above. Alower limit of the slider 1103 corresponds to the imaging range Rmin ofthe anterior eye 100-1. The slider 1203 is intended to perform an OCTfocus adjustment. The OCT focus adjustment is an adjustment for movingthe lens 115 in the direction indicated by an arrow illustrated in FIG.2 to make a focusing adjustment with respect to the anterior eye 100-1.The sliders 1103 and 1203 are also configured to move in an interlockingmanner with alignment operations performed in the respective images byusing the mouse. More specifically, the control unit (also referred toas a “focus interlocking unit”) of the personal computer 925 causes theOCT focus lens 115 to perform focusing on the anterior eye 100-1 in aninterlocking manner with the change of the optical path length of themeasurement light by the measurement light optical path length changingunit, either independent of or in an interlocking manner with the OCTfocus adjustment by the slider 1203. The focusing operation of theanterior eye observation unit on the anterior eye 100-1 needs to beperformed according to a change in the optical path length of themeasurement light, which is accompanied by a change in the imagingdistance P0. In the present exemplary embodiment, the foregoing controlunit (also referred to as an “anterior eye focusing interlocking unit”)causes the anterior eye observation unit to perform focusing on theanterior eye 100-1 in an interlocking manner with the change of theoptical path length of the measurement light by the measurement lightoptical path length changing unit.

FIG. 6 illustrates the measurement operation screen 1000 in which theslider 1103 illustrated in FIG. 5 is replaced with imaging rangeselection buttons 1004. Settings include a standard (R0=6 mm×6 mm), amaximum (Rmax=9 mm×9 mm), and a minimum (Rmin=3 mm×3 mm). If theoperator selects any one of the imaging range selection buttons 1004,the optical tomographic imaging apparatus can change the size of theimaging range of a tomographic image accordingly. The opticaltomographic imaging apparatus can change the size of the imaging rangeeven if the operator makes such a selection without an anterior eyeimage 1102 obtained.

[Flow for Obtaining Tomographic Image of Anterior Eye]

A flow for obtaining a tomographic image of an anterior eye 100-1 byusing the OCT apparatus according to the present exemplary embodimentwill be described with reference to FIG. 7. FIG. 7 is a flowchartillustrating a measurement flow according to the present exemplaryembodiment. The flowchart illustrates operations of the operator and thepersonal computer 925.

In step S101, the personal computer 925 starts the present measurementflow. In step S102, the optical tomographic imaging apparatus obtains ananterior eye image 1102 according to an instruction from the personalcomputer 925. The subject's eye 100 is illuminated with not-illustratedanterior eye illumination light. Reflected light passes through theobject lenses 101-1 and 101-2 and the optical path L2 described above,and forms an image on the CCD 105. The anterior eye image 1102 formed onthe CCD 105 is read by a not-illustrated CCD control unit, amplified,subjected to analog-to-digital (A/D) conversion, and input to acalculation unit. The anterior eye image 1102 input to the calculationunit is taken into the personal computer 925.

In step S103, the operator issues an instruction to the slider 1103 tochange the size of the imaging range to a desired size by using theinput unit 929, which issues instructions to the personal computer 925.A bar of the slider 1103 moves on-screen. According to the operator'sinstruction, the personal computer 925 serving as an example of thecontrol unit moves the optical head 900 in the optical axis direction toa distance corresponding to the changed size. In step S104, the personalcomputer 925 serving as an example of the control unit performs controlto move the reference mirror 119, according to the movement of theoptical head 900, to a position corresponding to the distance to whichthe optical path length of the measurement light is changed. As aresult, a coherence gate is adjusted so that an anterior eye tomographicimage is located within an imaging frame. The personal computer 925 maymove the lens 107 along with the movement of the reference mirror 119.When the personal computer 925 moves the optical head 900 and thereference mirror 119 in an interlocking manner according to theinstruction to change the size of the imaging range, the personalcomputer 925 may also move the OCT focus lens 115 in an interlockingmanner to change the focusing position. Instead of moving the referencemirror 119 in an interlocking manner, the personal computer 925 may movethe OCT focus lens 115 in an interlocking manner. In such a case, stepS106 to be described below may be omitted. The personal computer 925 maysimultaneously move such members. The personal computer 925 may movesuch members with a time difference.

In step S105, the personal computer 925 serving as an example of thecontrol unit moves the optical head 900 with respect to the anterior eye100-1 according to instructions from the operator, thereby performingalignment of the optical head 900 with respect to the anterior eye100-1. The alignment may be performed by moving the subject's facesupport with respect to the optical head 900. Aside from the operator'smanual operations, the optical head 900 may move automatically.Specifically, the personal computer 925 detects a pupil position of thesubject's eye 100 by image processing from the anterior eye image 1102captured by the CCD 105. Based on the detected pupil position, thepersonal computer 925 can find out an alignment position relationshipbetween the optical tomographic imaging apparatus and the subject's eye100. The personal computer 925 can drive the optical head 900 by using anot-illustrated XYZ stage so that the detected pupil position of thesubject's eye 100 comes to an ideal position. The personal computer 925may keep track of the anterior eye 100-1 while capturing a tomographicimage. In such a case, the operator can continue monitoring the anterioreye 100-1 of the subject's eye 100 with improved convenience.

In step S106, the operator issues an instruction to the slider 1203 tochange the focusing position of the anterior eye tomographic image byusing the input unit 929. A bar of the slider 1203 moves on-screen.According to the operator's instruction, the personal computer 925serving as an example of the control unit performs control to move theOCT focus lens 115. In such a manner, an OCT focus can be adjusted. Instep S107, the operator presses a capture button 1005 by using the inputunit 929. According to the operator's instruction, the personal computer925 serving as an example of the control unit performs control to obtaina tomographic image of the anterior eye 100-1. In step S108, thepersonal computer 925, serving as an example of a display control unit,causes the monitor 928 to display the tomographic image of the anterioreye 100-1. In step S108, the personal computer 925 may correct thetomographic image of the anterior eye 100-1 and cause the monitor 928 todisplay the corrected tomographic image. In step S109, the personalcomputer 925 ends the present measurement flow.

Note that the tomographic image obtained in step S107 may include awider or narrower range of regions than, for example, a tomographicimage obtained at the standard imaging distance P0 does in the screen ofthe same size. As will be described below, the correction is anoperation for enlarging or reducing a display range (angle of view) sothat the regions included in such captured images are displayed in thesame size as that of the region obtained at the imaging distance P0. Theabove operation is performed by a module area of the personal computer925, the module area functioning as an image correction unit forcorrecting and changing a display mode of an image. A module areafunctioning as the display control unit, which is included in thecontrol unit, displays a cursor or a display pattern for issuing aninstruction to change the imaging range on the display unit.

If the imaging distance P0 is greater than the standard imagingdistance, the tomographic image of the anterior eye 100-1 becomesnarrower only in the lateral direction without a change in tomographicdepth. If the imaging distance P0 is smaller than the standard imagingdistance, the tomographic image becomes wider only in the lateraldirection without a change in the tomographic depth. FIGS. 8A, 8B, 8C,8D, 8E, and 8F illustrate examples where display images of tomographicimages of an anterior eye 100-1 are corrected. FIG. 8A illustrates atomographic image of the anterior eye 100-1 with a lateral field of viewx0 corresponding to the imaging distance P0. If the imaging distance P0increases to Pmax, the lateral field of view x0 decreases to xm asillustrated in FIG. 8C. As illustrated in FIG. 8D, the lateral field ofview xm can be easily converted into the field of view x0 and displayedby using a known image processing method. A tomographic imagecorresponding to the imaging distance Pmin can be similarly processedand displayed as illustrated in FIG. 8F. Various measurements may beperformed based on the tomographic images illustrated in FIGS. 8D and8F. Various measurements may be performed by using the original imagesillustrated in FIGS. 8C and 8E, multiplied by the respective ratios ofthe imaging distances P and the lateral fields of view X.

As described above, the optical tomographic imaging apparatus accordingto the present exemplary embodiment can provide an apparatus with whichthe operator can specify various imaging ranges and capture images. Inother words, an optical tomographic imaging apparatus having variousfields of view and high resolution can be provided while maintaining theperformance of the optical systems. Since the operating distance betweenthe subject's eye 100 and the optical tomographic imaging apparatus canbe changed, burdens on the subject can be relieved by capturing an imagewith an increased operating distance according to the subject'scondition.

[Reducing Changing Speed of Optical Path Length when Reducing OpticalPath Length of Measurement Light]

An optical tomographic imaging apparatus (OCT apparatus) according to asecond exemplary embodiment will be described with reference to FIG. 9.In the first exemplary embodiment, the optical head 900 can be broughtclose to the subject's eye 100 to reduce the imaging distance P0 andthereby reduce the optical path length of the measurement light. Sincethe optical head 900 approaches the subject's eye 100, the possibilityof contact between the optical head 900 and the subject's eye 100increases. In the present exemplary embodiment, if an instruction tonarrow the imaging range of a tomographic image (obtain an enlargedimage) is issued, the personal computer 925, serving as an example of acontrol unit, controls the measurement light optical path lengthchanging unit to reduce the optical path length of the measurement lightand make a changing speed of the optical path length of the measurementlight lower than if an instruction to widen the imaging range (obtain awide-angle image) is issued. For example, to reduce the possibility ofcontact between the optical head 900 and the subject's eye 100, thepersonal computer 925 makes a moving speed of the optical head 900 lowerwhen the optical head 900 approaches the subject's eye 100 than when theoptical head 900 is moved away from the subject's eye 100.

Specifically, suppose that the input unit 929 (which is an example of aninstruction unit and may also be referred to as an operation unit) forissuing instructions to the personal computer 925 issues an instructionto narrow the imaging range of a tomographic image. In such a case, thepersonal computer 925, serving as an example of a control unit, controlsthe measurement light optical path length changing unit to make thechanging speed of the optical path length of the measurement light lowerthan if an instruction to widen the imaging range is issued, and reducethe optical path length of the measurement light. As a result, while theoptical head 900 is moving, the operator can easily determine whetherthe optical head 900 possibly comes into contact with the subject's eye100. Thus, safety can be enhanced.

As illustrated in FIG. 9, a cancel button 913 may be displayed near theslider 1103 so that if the operator determines that there is apossibility of contact between the optical head 900 and the subject'seye 100, the operator can cancel the movement of the optical head 900.Since the cancel button 913 is located near a position for issuing aninstruction to reduce the slider 1103, the operator can immediatelycancel the movement of the optical head 900. The cancel button 913 maybe a button for returning to a predetermined imaging distance P. If theoptical head 900 comes too close to the subject's eye 100, contact maybe avoided by moving the optical head 900 to a predetermined positionwhere the optical head 900 will not make contact with the subject's eye100.

Alternatively, before reducing the optical path length of themeasurement light, the optical tomographic imaging apparatus may onceobtain a wide-range tomographic image of the subject's eye 100 in aposition where the optical path length of the measurement light islarge, i.e., where the optical tomographic imaging apparatus is far fromthe subject's eye 100. Then, the operator may designate a narrowdetailed imaging range to move the optical tomographic imaging apparatuscloser to the subject's eye 100. Specifically, in step S102 of FIG. 7,the personal computer 925 sets the imaging distance P0 to a distancegreater than the standard imaging distance P0, i.e., a position fartherfrom the subject's eye 100. The optical tomographic imaging apparatuscan capture an image at the position farther from the subject's eye 100to obtain a wide-angle image of the anterior eye 100-1 as the anterioreye image 1102. As a result, the operator can check a wide range of theanterior eye 100-1. In step S103, the operator specifies an imagingrange. In such a manner, a detailed range can be imaged in fewer steps.

[Displaying Maximum Imaging Range and Minimum Imaging Range in Advance]

An optical tomographic imaging apparatus (OCT apparatus) according to athird exemplary embodiment will be described with reference to FIG. 9.In the first exemplary embodiment, the slider 1103 is intended tospecify the imaging distance to the subject's eye 100. The character1003 in the anterior eye observation screen 1101 is configured to changein size in an interlocking manner with the movement of the slider 1103.In other words, the operator can identify an imaging distance P from theposition of the slider 1103 and/or the size of the character 1003.Meanwhile, the imaging range has predetermined settings such as thestandard (R0=6 mm×6 mm), the maximum (Rmax=9 mm×9 mm), and the minimum(Rmin=3 mm×3 mm).

In the present exemplary embodiment, such predetermined imaging rangesare displayed on the same measurement operation screen 1000 as theslider 1103 is, like a position 906 corresponding to an enlargementlimit range 909 and a position 907 corresponding to a reduction limitrange 908. The operator can thus be informed of limits of the size ofthe imaging range. The limits of the size may also be displayed on thesame anterior eye observation screen 1101 as the character 1003 is, likethe reduction limit range 908 and the enlargement limit range 909. Thereduction limit range 908 is an imaging range corresponding to themaximum imaging distance Pmax. The maximum imaging distance Pmax isdetermined based on driving limits of the OCT focus lens 115 and thereference mirror 119. The enlargement limit range 909 is an imagingrange corresponding to the minimum imaging distance Pmin. The minimumimaging distance Pmin is determined based on the driving limits of theOCT focus lens 115 and the reference mirror 119, as well as a distanceat which safety to the subject's eye 100 can be secured from a viewpointof the contact of the optical head 900 with the subject's eye 100. Thereduction limit range 908 and the enlargement limit range 909 areexamples of a display pattern indicating a range of limits of theimaging range.

The reduction limit range 908 and the enlargement limit range 909 may bedisplayed by using different line types and/or colors from those of theslider 1103 and the character 1003. Such a display can improvevisibility to the operator. As described above, the enlargement limitrange 909 shows a state where the optical head 900 comes closest to thesubject's eye 100. From a viewpoint of contact between the optical head900 and the subject's eye 100, the enlargement limit range 909 can bedisplayed in red and/or in a thick line which have higher visibilitythan the reduction limit range 908. Since the limits of enlargement andreduction are displayed on-screen, the operator can identify theimageable range in advance. Suppose that the operator issues aninstruction to change the size of the imaging range outside the range oflimits of the imaging range on the display pattern (for example, theslider 1103) for issuing an instruction to change the size of theimaging range. In such a case, the personal computer 925, serving as anexample of a control unit, can cause the display unit to display adisplay pattern indicating a warning. Examples of outside the range oflimits of the imaging range include above the position 906 of the slider1103 corresponding to the enlargement limit range 909 and below theposition 907 of the slider 1103 corresponding to the reduction limitrange 908. Examples of the display pattern indicating a warning includea message indicating that the size of the imaging range cannot bechanged.

[Displaying Preview Image to Make Determination Before StartingMovement]

An optical tomographic imaging apparatus (OCT apparatus) according to afourth exemplary embodiment will be described with reference to FIG. 9.In the first exemplary embodiment, the operator determines the imagingrange by using the slider 1103 or the imaging range selection buttons1004 on the measurement operation screen 1000. In the present exemplaryembodiment, in step S103 of FIG. 7, the operator specifies the imagingrange. The personal computer 925 then displays preview screens 910 sothat the operator can check preview images 911 and 912. The previewimages 911 and 912 illustrated in FIG. 9 are an enlarged preview imageand a reduced preview image, respectively. The personal computer 925generates the preview images 911 and 912 without changing the imagingdistance P, but by trimming image data and/or performing processing forchanging the display magnification based on the already obtainedanterior eye observation image 1102. The preview images 911 and 912 havelower image quality than that of images that are supposed to beobtained. The areas outside the reduced preview image 912 are notobtained yet. The outside areas may be displayed in black. The originalanterior eye observation image 1102 may be blurred and displayed in theoutside areas.

If the operator checks the preview screens 910 and determines thateither one of the imaging ranges is desired, the operator determines byusing the corresponding determination button 905 to actually enlarge orreduce the imaging range. After the determination, the control unitmoves the optical head 900 to change the imaging distance P. Accordingto the present exemplary embodiment, a delay due to erroneousspecification of the imaging range can be avoided.

The present invention is not limited to the foregoing exemplaryembodiments, and various changes and modifications may be made withoutdeparting from the gist of the foregoing exemplary embodiments. Forexample, while the foregoing exemplary embodiments have dealt with thecase where the object to be measured is the eye, the exemplaryembodiments may be applied to objects to be measured other than the eye.Examples include the skin and organs. In such cases, the exemplaryembodiments of the present invention are configured as medicalapparatuses other than an ophthalmologic apparatus, such as anendoscope. The subject's eye described above can thus be regarded as anobject.

Embodiments of the present invention can also be realized by a computerof a system or apparatus that reads out and executes computer executableinstructions recorded on a storage medium (e.g., non-transitorycomputer-readable storage medium) to perform the functions of one ormore of the above-described embodiment(s) of the present invention, andby a method performed by the computer of the system or apparatus by, forexample, reading out and executing the computer executable instructionsfrom the storage medium to perform the functions of one or more of theabove-described embodiment(s). The computer may comprise one or more ofa central processing unit (CPU), micro processing unit (MPU), or othercircuitry, and may include a network of separate computers or separatecomputer processors. The computer executable instructions may beprovided to the computer, for example, from a network or the storagemedium. The storage medium may include, for example, one or more of ahard disk, a random-access memory (RAM), a read only memory (ROM), astorage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2013-017657 filed Jan. 31, 2013, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An optical tomographic imaging apparatusconfigured to obtain a tomographic image of an anterior eye based onlight into which return light from the anterior eye irradiated withmeasurement light and reference light corresponding to the measurementlight are combined, the optical tomographic imaging apparatuscomprising: a measurement light optical path length changing unitconfigured to change an optical path length of the measurement light; aninstruction unit configured to input an instruction on a size of animaging range of the tomographic image; and a control unit configuredto, in a case where an instruction to narrow the imaging range is input,control the measurement light optical path length changing unit toreduce the optical path length of the measurement light and make achanging speed of the optical path length of the measurement light lowerthan in a case where an instruction to widen the imaging range is input.2. The optical tomographic imaging apparatus according to claim 1,wherein the measurement light optical path length changing unit includesan optical unit moving mechanism configured to move an optical unitincluding an optical path of the measurement light with respect to theanterior eye, and wherein the control unit is configured to, in a casewhere an instruction to narrow the imaging range is input, control theoptical unit moving mechanism to bring the optical unit closer to theanterior eye and make a moving speed of the optical unit lower than in acase where an instruction to widen the imaging range is input.
 3. Theoptical tomographic imaging apparatus according to claim 1, furthercomprising a display control unit configured to cause a display unit todisplay a display pattern indicating a range of limits of the imagingrange.
 4. The optical tomographic imaging apparatus according to claim3, wherein the display control unit is configured to cause the displayunit to display a display pattern indicating an enlargement limit rangeof the imaging range and a display pattern indicating a reduction limitrange of the image range as superimposed on an image of the anterioreye, and wherein the display pattern indicating the enlargement limitrange is a display pattern having higher visibility than that of thedisplay pattern indicating the reduction limit range.
 5. The opticaltomographic imaging apparatus according to claim 3, wherein the displaycontrol unit is configured to cause the display unit to display adisplay pattern for inputting an instruction on a change of the size ofthe imaging range of the tomographic image, and wherein the instructionunit is configured to input an instruction on the change according to anoperation by an operation unit.
 6. The optical tomographic imagingapparatus according to claim 5, wherein the operation unit includes amouse.
 7. The optical tomographic imaging apparatus according to claim5, wherein the display control unit is configured to, in a case where aninstruction on the change is input outside the range of limits of theimaging range on the display pattern for inputting an instruction on thechange, cause the display unit to display a display pattern indicating awarning.
 8. The optical tomographic imaging apparatus according to claim1, further comprising a reference light optical path length changingunit configured to change an optical path length of the reference light,wherein the control unit is configured to control the measurement lightoptical path length changing unit and the reference light optical pathlength changing unit in an interlocking manner according to theinstruction by the instruction unit.
 9. The optical tomographic imagingapparatus according to claim 1, further comprising a moving unitconfigured to move a focusing lens along an optical path, the focusinglens focusing the measurement light on the anterior eye, wherein thecontrol unit is configured to control the measurement light optical pathlength changing unit and the moving unit in an interlocking manneraccording to the instruction by the instruction unit.
 10. A method forcontrolling an optical tomographic imaging apparatus configured toobtain a tomographic image of an anterior eye based on light into whichreturn light from the anterior eye irradiated with measurement light andreference light corresponding to the measurement light are combined, themethod comprising: inputting an instruction on a size of an imagingrange of the tomographic image; and controlling, in a case where aninstruction to narrow the imaging range is input, a measurement lightoptical path length changing unit to reduce an optical path length ofthe measurement light and make a changing speed of the optical pathlength of the measurement light lower than in a case where aninstruction to widen the imaging range is input, the measurement lightoptical path length changing unit being configured to change the opticalpath length of the measurement light.
 11. The method according to claim10, wherein the measurement light optical path length changing unitincludes an optical unit moving mechanism configured to move an opticalunit including an optical path of the measurement light with respect tothe anterior eye, and wherein the method further comprises controlling,in a case where an instruction to narrow the imaging range is input, theoptical path moving mechanism to bring the optical unit closer to theanterior eye and make a moving speed of the optical unit lower than in acase where an instruction to widen the imaging range is input.
 12. Themethod according to claim 10, further comprising causing a display unitto display a display pattern indicating a range of limits of the imagingrange.
 13. The method according to claim 12, wherein the method furthercomprises causing the display unit to display a display patternindicating an enlargement limit range of the imaging range and a displaypattern indicating a reduction limit range of the imaging range assuperimposed on an image of the anterior eye, and wherein the displaypattern indicating the enlargement limit range is a display patternhaving higher visibility than that of the display pattern indicating thereduction limit range.
 14. The method according to claim 12, furthercomprising: causing the display unit to display a display pattern forinputting an instruction on a change of the size of the imaging range ofthe tomographic image; and inputting an instruction on the changeaccording to an operation by an operation unit.
 15. The method accordingto claim 14, wherein the operation unit includes a mouse.
 16. The methodaccording to claim 14, further comprising causing, in a case where aninstruction on the change is input outside the range of limits of theimaging range on the display pattern for inputting an instruction on thechange, the display unit to display a display pattern indicating awarning.
 17. The method according to claim 10, further comprisingcontrolling the measurement light optical path length changing unit anda reference light optical path length changing unit in an interlockingmanner according to the instruction, the reference light optical pathlength changing unit being configured to change an optical path lengthof the reference light.
 18. A non-transitory computer-readable storagemedium storing a program that causes a computer to perform the methodaccording to claim
 10. 19. The optical tomographic imaging apparatusaccording to claim 1, further comprising a scanning unit configured toscan the anterior eye with the measurement light, wherein the controlunit is configured to control the scanning unit so as not to change ascan angle of the scanning unit.
 20. The method according to claim 10,further comprising scanning, by a scanning unit, the anterior eye withthe measurement light, wherein the scanning unit is controlled so as notto change a scan angle of the scanning unit.
 21. The optical tomographicimaging apparatus according to claim 1, further comprising an objectivelens arranged in an optical path of the measurement light, wherein themeasurement light optical path length changing unit is configured tochange the optical path length of the measurement light by changing adistance between the objective lens and the anterior eye.
 22. The methodaccording to claim 10, wherein the measurement light optical path lengthchanging unit is configured to change the optical path length of themeasurement light by changing a distance between the objective lens andthe anterior eye, and wherein the objective lens is arranged in anoptical path of the measurement light.